1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching liquid crystal display (IPS-LCD) device and a manufacturing method of the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have an alignment direction as a result of their long thin shapes and can be arranged to have initial pretilt angles. The alignment direction can be controlled or augmented by applying an electric field. More specifically, the intensity of an applied electric field varies the alignment of the liquid crystal molecules. Due to the optical anisotropy of liquid crystal molecules, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by controlling the applied electric field, an image that has a desired brightness can be produced. Among the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and capability to display moving images.
In general, a liquid crystal display (LCD) device includes two substrates, which are spaced apart and face each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode, and the electrodes of each substrate also face each other. A voltage is applied between the electrodes such that an electric field is induced between the electrodes. The alignment direction of the liquid crystal molecules between the electrodes is changed by varying the intensity of the electric field. Since the electrodes are positioned on the two substrates, respectively, the electric field induced between the electrodes is perpendicular to the lower and upper substrates. Accordingly, the related art LCD devices have a narrow viewing angle because of the longitudinal electric field control of the liquid crystal molecules. To solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been developed. An IPS-LCD device includes a pixel electrode and a common electrode on the same substrate.
FIG. 1 is a cross-sectional view illustrating an in-plane switching liquid crystal display (IPS-LCD) device according to the related art. As shown in FIG. 1, the IPS-LCD device includes an array substrate and a color filter substrate with a liquid crystal layer LC interposed therebetween. A pixel region P is defined on a first substrate 50. A thin film transistor T is formed within the pixel region P on the first substrate 50. The thin film transistor T includes a gate electrode 52, a semiconductor layer 62, a source electrode 64, and a drain electrode 66. Further, a common electrode 58 and a pixel electrode 72 are also formed within the pixel region P. The common electrode 58 and the pixel electrode 72 are spaced apart from each other on the same substrate.
The common electrode 58 is formed of the same material and in the same layer as the gate electrode 52, and the pixel electrode 72 can be formed of the same material and in the same layer as the source and drain electrodes 64 and 66. The pixel electrode 72 can be formed of a transparent conductive material to improve the aperture ratio. Although not shown in FIG. 1, a gate line is formed along a side of the pixel region P and a data line is formed along a direction perpendicular to the gate line. A common line may be formed parallel to the gate line, and the common line provides voltages to the common electrode 58.
A second substrate 30 is spaced apart from the first substrate 50. A black matrix 32 is formed on an inner surface of the second substrate 30, i.e., a surface facing the first substrate 50. The black matrix 32 corresponds to the gate line (not shown), the data line (not shown) and the thin film transistor T. A color filter layer is also formed on the inner surface of the second substrate 30, and the color filter layer includes three color filters of red 34a, green 34b, and blue (not shown). The color filter layer corresponds to the pixel region P.
A liquid crystal layer LC is interposed between the first substrate 50 and the second substrate 30. Liquid crystal molecules of the liquid crystal layer LC are oriented in response to a horizontal electric field 95 induced between the common electrode 58 and the pixel electrode 72. Light transmission through the liquid crystal layer LC depends upon the orientation of the liquid crystal molecules.
FIG. 2 is a plan view illustrating one sub-pixel of an array substrate for an IPS-LCD device according to the related art. As shown in FIG. 2, a gate line 54 is formed on a substrate 50 along a first direction. A data line 68 is formed on the substrate 50 along a second direction perpendicular to the first direction. The gate line 54 and the data line 68 cross each other to define a pixel region P. A common line 56 is spaced apart from and parallel to the gate line 54. A thin film transistor T is formed adjacent to a crossing of the gate line 54 and the data line 68. The thin film transistor T includes a gate electrode 52, a semiconductor layer 62, a source electrode 64, and a drain electrode 66.
A pixel electrode 72 is formed in the pixel region P and is connected to the drain electrode 66. The pixel electrode 72 includes finger portions parallel to the data line 68. A common electrode 58 is also formed in the pixel region P and is connected to the common line 56. The common electrode 58 includes finger portions parallel to the data line 68 and spaced apart from each other. The portions of the common electrode 58 are interleaved with the portions of the pixel electrode 72. Both the common electrode 58 and the pixel electrode 72 are formed of a transparent conductive material. Even though the common electrode 58 and the pixel electrode 72 are transparent, the light transmittance is not remarkably increased as compared to common and pixel electrodes formed of an opaque conductive material.
FIG. 3 is a cross-sectional view along the line III-III of FIG. 2 together with a transmittance curve F corresponding to the electrodes. As shown in FIG. 3, a gate insulating layer GL is formed on a substrate 50, where a pixel region P is defined, and a data line 68 is formed on the gate insulating layer GL at a side of the pixel region P. A passivation layer PL is formed on the data line 68. A common electrode 58 and a pixel electrode 72 are formed on the passivation layer PL in the pixel region P. The common electrode 58 and the pixel electrode 72 are spaced apart from each other and interleaved with each other. The common electrode 58 and the pixel electrode 72 are formed of a transparent conductive material.
When a voltage is applied to the common electrode 58 and the pixel electrode 72, an electric field 95 is induced between the common electrode 58 and the pixel electrode 72. The electric field 95 only spreads several micrometers into the electrodes from edges of the electrodes. That is, since the electric field 95 does not significantly occur or is not present at central portions of the common electrode 58 and the pixel electrode 72, liquid crystal molecules (not shown) over the central portions of the common electrode 58 and the pixel electrode 72 are not affected by an electric field 95.
As shown in FIG. 3, just small areas K along both sides of each of the common electrode 58 and the pixel electrode 72 are influenced by the electric field 95. The intensity of the electric field 95 in the areas K is much weaker than that in the region between the common electrode 58 and the pixel electrode 72. Accordingly, the light transmittance in the areas K goes down rapidly as compared with the region between the common electrode 58 and the pixel electrode 72. Although the common and pixel electrodes 58 and 72 are transparent, there is little improvement in the brightness of the display because the liquid crystal directly over the common electrode 58 and the pixel electrode 72 is not actuated because of the lack of an electric field.